1. Field of the Invention
The present invention relates to a swash plate type hydraulic motor switchable between high speed and low speed, including, a main ease, a cylinder block supported in the main ease to be rotatable about a rotational axis, the cylinder block supporting a plurality of plungers having longitudinal axes extending parallel to the rotational axis, a swash plate supported in the main case for contacting forward ends of the plungers and pivotable about a pivotal axis between a high speed position and a low speed position, the pivotal axis extending parallel to a plane perpendicular to the rotational axis, and a swash plate angle switching device for switching the swash plate to one of the high speed position and low speed position.
2. Description of the Related Art
A hydraulic motor switchable between high speed and low speed as noted above is disclosed in U.S. Pat. No. 4,690,036, for example. This hydraulic motor includes a swash plate angle adjusting device having spherical elements mounted in a deep inward region of a drive chamber defined by the main case, for engaging and supporting the swash plate. The swash plate is pivotable about a transverse axis of the spherical elements. The main ease includes a hydraulic cylinder for pressing the swash plate at a side facing away from the plungers to tilt the swash plate.
In this swash plate angle adjusting structure, the swash plate is engaged and supported only by the spherical elements in the deep end of the drive chamber. No means is provided to inhibit the swash plate from lifting or becoming loose toward the cylinder block. When the plungers of the cylinder block exert a smaller pressing force than the swash plate angle varying hydraulic cylinder, the swash plate could become loose from the spherical elements, thereby to be pivotable in an unsteady way. To avoid such an inconvenience in practice, the pivotal axis of the swash plate must be offset to a large extent from a rotational axis of the motor so that, based on the pressing force of the plungers of the cylinder block, a sufficient tilting moment is constantly applied to the swash plate in a fixed direction (toward the low speed position).
However, in the structure in which the pivotal axis of the swash plate is sufficiently offset from the rotational axis of the motor to prevent the swash plate from becoming loose, the hydraulic cylinder must apply a strong pressing force to cause the swash plate to pivot to the high speed position. This requires the hydraulic cylinder to have a large diameter. In addition, complicated setting of hydraulic circuitry is required, such as for suitably balancing the pressing forces of the plungers and hydraulic cylinder, in order to stabilize postures of the swash plate. To meet his requirement, the components must have a high degree of precision, which has been a cause of the high manufacturing cost.